How come I can’t remove the blue textarea border in Twitter Bootstrap?

You have write -webkit-appearance:none; like this:

textarea:hover, 
input:hover, 
textarea:active, 
input:active, 
textarea:focus, 
input:focus,
button:focus,
button:active,
button:hover,
label:focus,
.btn:active,
.btn.active
{
    outline:0px !important;
    -webkit-appearance:none;
    box-shadow: none !important;
}